1. Field of the Invention
The present invention relates to a resin composition for powder coatings.
2. Description of the Prior Art
With the recent development of the technique of electrostatic powder coating of objects, powder coating compositions have been noted as being valuable. Powder coatings have various advantages. For example, unlike solvent-type paints, powder coatings do not contain volatile solvents at all and hence, they do not cause environmental pollution. The coated articles can be used immediately after coating and a thick coating can be obtained by one coating operation. Because of these advantages, the use of powder coatings has been rapidly increasing in recent years.
As resins for powder coatings, there have heretofore been used thermoplastic resins such as polyvinyl chloride, nylon and polypropylene, and thermosetting resins such as epoxy resins, thermosetting polyester resins and thermosetting acrylic resins. In general, thermoplastic resins are disadvantageous because the use of a primer is indispensable and the properties of the resulting coatings are insufficient. Accordingly, investigations have been made vigorously to develop powder coatings comprising thermosetting resins.
Epoxy resins having thermosetting characteristics provide coatings which are excellent in corrosion resistance, water resistance, adhesion to metals and mechanical strength, but coatings made from a powder coating composition comprising an epoxy resin are inferior in weatherability. As a thermosetting resin capable of overcoming this disadvantage, there can be mentioned a thermosetting acrylic resin, and it provides coatings possessing excellent weatherability and surface smoothness. Thermosetting acrylic resins are used in powder coatings for painting automobiles and household electric appliances. However, the thermosetting acrylic resin is inferior to the epoxy resin in corrosion resistance and adhesion to metals. Most of the thermosetting polyester resins have properties intermediate between the properties of epoxy and thermosetting acrylic resins, but because a good balance of the various required properties of coatings cannot be attained with thermosetting polyester resins, the utilization thereof has lagged behind the use of other resins. Thus, various powder coatings have advantages and disadvantages and powder coating compositions having fully satisfactory properties have not been developed.